# upgrade LED Trailer lights for 24V Use

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#### Seedz

##### Newbie level 2
Hi Guys,

this should be an easy one but i can't seem to figure it out:

I've got these LED trailer lights that i bought from China (there's the problem ;-) that were described as 12v/24v compatible but after testing then on my regulated power supply on 24V they didn't last very long:

It's a very basic pcb design consisting of 16 LED's which are wired in series and parallel with some resistors and 2 diodes.
After researching it a bit it is obvious that this is designed around 12Volt input and would have never lasting long on the 24 volt motorhome it is intended for.

Does anyone know a quick way in changing the resistors and or diode so that i can withstand a constant voltage of 27Volt (that's what i got when the alternator is running)

See Attachment for pcd layout.

The pcd consist of 2 circuits where the constant voltage on the red wire would give you the "tail lights" which is limited by a resistor and the green light would bypass the resistor and provide the LED's with a higher voltage that gives you the "brake" lights.

So my first question is:
what is a safe voltage for 1 LED to have when i'm braking and my input voltage is 27Volts?

i tried to work it out and got this far:

R = E/I
E = Vbatt - Vled
I = 20mA

Vled = 2.1 volts
Vbatt = 27 volts

R=24.9/0.02 = 1245 ohms

The resistors that are on it at the moment are 100 Ohm and 570 Ohm in parallel creating effectively 85 Ohms over the circuit.

What resistors should i use to find the best range to drive these LED as a "tail light" and "brake light" circuit.

sorry for the non technical English.
Sietse

#### Attachments

• back.jpg
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• front.jpg
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#### ArticCynda

To redimension the resistors, we have to assume that the circuit is designed run at 12V since we don't have the datasheet of the LEDs to determine an appropriate value for forward current.

Hook the circuit to 12V and measure the voltage over the series resistors. Now divide this voltage $V_R$ by the resistor value, that gives you the LED forward current $I_F$. The resistor value for 27V can now be calculated as follows:
$R_{27V} = \frac{27 - (12 - V_R)}{I_F}$
This is nothing else than applying Ohm's Law.

#### Audioguru

It is illegal in many countries to tamper with vehicle lighting. Yours will probably be too dim, especially off axis. Your trailer will be a hazzard.

#### ArticCynda

It is illegal in many countries to tamper with vehicle lighting. Yours will probably be too dim, especially off axis. Your trailer will be a hazzard.
Please cite your source(s). In Belgium, it is perfectly legal to modify your vehicle any way you want, including lighting, as long as it passes a mandatory check every few years.

The "tampering" here is replacing a resistor with another one. To detect this modification, officials would have to be able to read resistor color codes. Highly unlikely I think.

#### Audioguru

Here in Canada, expensive but cheaply made lighting kits for cars are sold. Young people replace the original lighting on their cars with them while lowering and making their cars noisy. It is obvious that the replacement lights are much too dim (some people also blacken them) and cannot be seen off to one side so the police impound the cars until the lighting is back to normal.

New cars have factory installed LED lighting but they do not use the general purpose 5mm 20mA LEDs you have. Your LEDs appear to have a lens that focuses the light into a bright but very narrow beam which is bad. I was given a big bag full of Luxeon Superflux LEDs that have 4 pins for cooling and operate up to 70mA. They have a very bright very wide angle light beam and are used in some cars.

Police stopped me and gave me a warning because the licence plate light bulb was burnt out, even though most roads have lighting and the numbers can be seen without a light bulb.

#### ArticCynda

Audioguru said:
Police stopped me and gave me a warning because the licence plate light bulb was burnt out, even though most roads have lighting and the numbers can be seen without a light bulb.
A harrowing example of how red tape is wasting time, money and resources in so called "developed countries".

#### Audioguru

A harrowing example of how red tape is wasting time, money and resources in so called "developed countries".
I am in a developed country but I guess I am not developed because I did not replace the burnt out light bulb with an LED that lasts "forever".

#### srizbf

Hi Guys,

this should be an easy one but i can't seem to figure it out:

....

It's a very basic pcb design consisting of 16 LED's which are wired in series and parallel with some resistors and 2 diodes.
After researching it a bit it is obvious that this is designed around 12Volt input and would have never lasting long on the 24 volt motorhome it is intended for.

Does anyone know a quick way ...

Sietse
can you connect all the LEDs in series and test it?(for meeting 24V input)

#### Audioguru

can you connect all the LEDs in series and test it?(for meeting 24V input)
That will not light the LEDs. He said they are 2.1V each. He also said his "24V" battery is actually 27V (or less as its voltage runs down). Then simple arithmetic shows that 16 LEDs need 33.6V that is not available.

#### Seedz

##### Newbie level 2

I don't quite understand why they put two resistors in this circuit in paralell in the first place.
i just de soldered the 100 ohm resistor and managed to put the circuit on 22 volts where the total current draw 40ma (that should be the max. right?)
with 4 LED's in two strings in parallel they would each draw 20mA

So if the voltage drop over the diode is 0.7V and over the LED is 2.1V then i end up with this:

R=27 - 0.7 - (4x 2.1) / 0.02 (20mA)
R= 26.3 - 8.1 / 0.02
R= 18.2 / 0.02 = 910 Ohm

is this correct?

what about the 2 different currents for tail light and brake light.

on the original circuit on 12volts i measure 17mA for tail light and 35mA for brake.
is 35mA not too much, even though you would only have the brakes on for a few seconds and at most 1 minute.
will it shorten the LED's lifespan?

i noticed that with the current setup when i start to raise the voltage there is not a very visual difference between tail lights and brake.

Thanks.

#### hotwaterwizard

##### Full Member level 3
I would run two sets in series and have twice as much light for each set.

- - - Updated - - -

- -l<l- + - -l<l- +

#### Audioguru

When you double the power of a light then it appears only a little brighter because our vision's sensitivity to light is logarithmic. A common light bulb at the rear of cars is a 3157. Brakes and turn signal are 27W and lights at night are 7W.

#### D.A.(Tony)Stewart

THis is a good example of a cheap design that costs about 50 cents to make in CHina. What did you pay?

REMOVE all 100 Ohm resistors for 27 V use. I assume you didn't get instructions.

THe middle Resistor is beyond rated power with taillight on only 380mW so will rather hot to touch and should use two 1K resistors or so in parallel for cooler operation.

Tail light total LED current is 26 mA divided equally in 4 serial strings or 6.5mA ea.
Brake light total LED current is 126mA " " " " or 31.6mA per LED , which is beyond rated current but OK for short durations . It will run the junctions up to 100'C in a few seconds, but for an hour a day stopped in rush hour traffic, it will work but not as long or as bright if using 50mA Ultrabright LED's with 4 leads.

But to make the design work as intended remove all 100 Ohm R's for 24~28V operation and leave in for 12~14V operation.

Red is brake +V switched and greenish is Tail light +V, while black is chassis ground. ( American convention)

The central located tail light resistor will be burning hot and dissipates 420mW driving all LEDs and looks to be either 1/8 or 1/4 W RATING. ( CHange with two 1K resistors might be better. It may work and will not live it's full life of 50+ yrs but might last as long as life of LEDs ( possibly 5~10yrs). ( who knows with this cheap quality how reliable these LEDs are)

LED Vf may go from 2.0 to 2.2V with brake switch ON
supplier tolerance unknown, they could be much higher.. i.e. 2.6V meaning cheaper quality.... but then cooler central R.....

Simulation tester

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